This application claims foreign priority under 35 U.S.C. xc2xa7119 from Italian Patent Application Serial Number M199A 001409 filed Jun. 24, 1999, incorporated herein by reference for all purposes.
1. The Field of the Invention
The present invention relates to getter devices that evaporate calcium to form a calcium film within vacuum systems, and particularly in cathode ray tubes (CRTs) and similar devices.
2. Background
Getter devices based on the evaporation of a metal are commonly known as evaporable getter devices. These devices have been in use since the 1950""s for maintaining the vacuum inside cathode ray tubes (also commonly referred to as kinescopes) of televisions and, later, within computer monitors. A CRT is evacuated during its manufacture, typically by means of a mechanical pump, and then hermetically sealed. However, the vacuum within the tube tends to decrease quickly, mainly due to the outgassing of components situated within the CRT. Therefore, getter materials capable of sorbing gas molecules have been used to preserve the required vacuum level necessary for proper CRT operation. Barium has been employed as such a: getter material, as is well known in the art. The high reactivity of barium in air, however, renders it difficult to handle in manufacturing operations, thus barium is frequently used in the form of the air stable compound BaAl4.
The getter material is placed within a CRT before it is sealed, typically by fritting, and then the getter is inductively heated by radiofrequency (RF) radiation from a RF source, such as an inductance coil, located outside of the CRT. The heating from the RF radiation is sufficient to evaporate barium that subsequently condenses as a film on the internal walls of the tube. The film then provides a very high surface area for gettering reactive gas species from the enclosed volume.
The getter material is commonly placed within some type of container prior to being sealed within a CRT both for ease of handling and to improve the evaporation process. The getter material is typically pressed into the container, and hereinafter a compressed getter material formed within a container will be referred to as a powder packet. The container can be as simple as a short cylinder open at one end. Other containers take the form of a metal disk or ring with an annular channel formed into one side for holding the getter material. Various container shapes are described in U.S. Pat. Nos. 2,842,640, 2,907,451, 3,033,354, 3,225,911, 3,381,805, 3,719,433, 4,134,041, 4,504,765, 4,486,686, 4,642,516 and 4,961,040, each incorporated herein by reference. Moreover, in order to impart greater homogeneity to the induction heating of the powder packet, a discontinuous metal element, disposed essentially parallel to the container bottom, can be placed within the packet itself as described in U.S. Pat. No. 3,558,962 and in European patent application EP-A-853328, both incorporated herein by reference.
Barium evaporation requires temperatures of about 1200xc2x0 C., and thus consumes considerable energy. However, it is well known in the art that when a powder of BaAl4 is mixed with a nickel powder and the mixture is heated to a temperature of about 850xc2x0 C., the following exothermic reaction takes place:
BaAl4+4Nixe2x86x92Ba+4NiAl
The heat generated by this exothermic reaction further raises the temperature of the system to that required for barium evaporation. Consequently, mixtures with powdered nickel require less heating from the outside in order to produce barium vapor.
Barium evaporable getters have been further improved, for example, by the addition of up to 5% by weight of a compound selected from amongst the group consisting of iron nitride, germanium nitride, nitrides of iron-germanium alloys, and mixtures thereof. In these devices nitrogen is released immediately before the calcium begins to evaporate, and the effect of the nitrogen is to create a more diffuse metal film having a more homogeneous thickness. Examples of nitrogenated devices for barium evaporation are given in U.S. Pat. Nos. 3,389,288 and 3,669,567 which are both incorporated herein by reference.
In order to better protect a device against atmospheric gasses, especially during the fritting operation referred to above, the powder, or some portion thereof, can be covered with a protective film. Such films are generally glassy layers comprised of boron oxide as the predominant or sole component. Getter devices completely covered by a thin film of a boron compound possibly containing silicon oxide up to 7% by weight are described in U.S. Pat. No. 4,342,662, incorporated herein by reference. Other getter devices in which at least the particles of nickel are protected by boron oxide are described in Japanese patent Hei-2-6185, incorporated herein by reference.
One problem associated with barium-based getter devices is that particles can be ejected during barium evaporation. U.S. Pat. No. 5,118,988, incorporated herein by reference, describes this problem and provides a solution through the use of radial depressions formed into the free surface of the powder in the container. Between two and eight, and typically four, such radial depressions are used to reduce heat transport in a circumferential direction within the powder to achieve the desired effect.
Other problems associated with the use of barium as a getter material, however, are more intractable. First, like all heavy metals, barium is a toxic element and therefore its use requires particular precautions in all production steps of the compound BaAl4, as well as in the disposal of CRTs to avoid ecological problems.: Further, where barium inside a CRT is hit by the high energy electron beam used to excite phosphors and generate an image, the barium will emit harmful X-rays that can escape from the CRT and pose an additional health hazard.
The article xe2x80x9cBarium, Strontium and Calcium as Getters in Electron Tubesxe2x80x9d (J. C. Turnbull, Journal of Vacuum Science and Technology, vol. 14, no. 1, January/February 1977, pp. 636-639) considers the possibility of replacing barium: with either strontium or calcium for applications in kinescopes. The strontium and calcium precursor materials used in this study are obtained by melting mixtures containing 40% of Sr and 60% of Al, and 35% of Ca and 65% of Al respectively, where all percentages are by weight. Analyses of the materials thus obtained show that in the first case the resulting material is a mixture of the compound SrAl4 with free Al, and in the second case is a complex mixture of phases, containing the compounds CaAl2, CaAl and CaO without free Al.
The results of the Turnbull study further show that it is possible to obtain a strontium film having gas sorption features comparable with those of barium film, while calcium gives a much poorer result. Particularly, the study shows that given the same weight of metal, a strontium film has a sorption capacity for oxygen that is 75% of that of a barium film, whereas the capacity of a calcium film is only 25% of that of the barium film. Confirming these results is U.S. Pat. No. 3,952,226 issued to Turnbull that describes the use of strontium-based evaporable getters to substitute for barium-based getters, but omits the possibility of employing similar calcium-based devices.
Additionally, it should be noted that world-wide production of CRTs has always been based exclusively on the use of barium as the getter film, and of the compound BaAl4 as the precursor to the film. It is clear, therefore, that despite all of the disadvantages of using barium as a getter material in CRTs, in half of a century no better alternative has yet been devised that is both economical and effective.
It is an object of the present invention, therefore, to provide a getter material that does not include barium yet has comparable sorption characteristics and that can be readily substituted for the barium precursors presently used in existing manufacturing processes for devices maintained under vacuum such as CRTs.
The present invention provides a getter device for maintaining a vacuum in a sealed enclosure. The device comprises a calcium-aluminum compound including about 39% to about 43% calcium by weight, and is capable of producing a calcium vapor when heated. The calcium vapor can subsequently condense to form a calcium film on an inside surface of the sealed enclosure that can getter reactive gases from inside the sealed enclosure. The calcium-aluminum compound is preferably CaAl2 as this compound is stable in air and therefore easier to store, handle, and use. The CaAl2 is preferably powdered with a particle size less than about 500 xcexcm and preferably between about 50 xcexcm and about 250 xcexcm. Calcium-based getter devices are desirable as an alternative to barium-based evaporable getters and provide a film with a gettering capacity that in some embodiments is superior on a per weight basis to that obtainable by a barium film.
The CaAl2 can also be mixed with a nickel powder, a titanium powder, or both, where the added metal has a particle size less than about 100 xcexcm and preferably between about 20 xcexcm and about 70 xcexcm. CaAl2-nickel mixtures can have a weight ratio of CaAl2 to nickel between about 20:80 and about 45:55 and preferably between about 38:62 and about 42:58. CaAl2-titanium mixtures can have a weight ratio of CaAl2 to titanium between about 40:60 and about 75:25 and preferably between about 45:55 and about 50:50. The CaAl2, either alone or mixed with an added metal, can further include up to about 4% by weight of a compound selected from amongst the group consisting of iron nitride, germanium nitride, nitrides of iron-germanium alloys, and mixtures thereof. Further still, one or more of the powders in the getter device can itself include a boron-based protective film in order to protect the powders against atmospheric gasses. The mixtures of the present invention are advantageous because the added metal reacts exothermically with the CaAl2 and therefore they require less inductive heating to produce a calcium vapor. The mixtures with nickel are further advantageous because there is almost no dependency between the supplied heating power and the amount of calcium vapor produced, and this is true even after the mixture has been exposed to oxidizing gases and high temperatures.
A getter device of the present invention can by used as a free powder or can be placed within an open container for easy of handling and better evaporation performance. When placed in an open container, the powder can either be loose or compacted. Either way, a powder in an open container will have a free surface facing the opening. The free surface of the powder can have at least two and as many as eight radial depressions to reduce the likelihood of particles being ejected during calcium evaporation. The getter device can further include a discontinuous metal element disposed essentially parallel to a bottom of the container in order to impart greater homogeneity to the induction heating of the powder.